02333cam a22002537 4500001000700000003000500007005001700012008004100029100002000070245009700090260006600187490004200253500001900295520125100314530006101565538007201626538003601698690007001734690008101804710004201885830007701927856003802004856003702042w10920NBER20150802201547.0150802s2004 mau||||fs|||| 000 0 eng d1 aNorberg, Karen.10aPartnership Status and the Human Sex Ratio at Birthh[electronic resource] /cKaren Norberg. aCambridge, Mass.bNational Bureau of Economic Researchc2004.1 aNBER working paper seriesvno. w10920 aNovember 2004.3 aIf two-parent care has different consequences for the reproductive success of sons and daughters, then natural selection may favor adjustment of the sex ratio at birth according to circumstances that forecast later family structure. In humans, this partnership status hypothesis predicts fewer sons among extra-pair conceptions, but the rival "attractiveness" hypothesis predicts more sons among extra-pair conceptions, and the "fixed phenotype" hypothesis predicts a constant probability of having a son, regardless of partnership status. In a sample of 86,436 human births pooled from five US population-based surveys, I find 51.5% male births reported by respondents who were living with a spouse or partner before the child's conception or birth, and 49.9% male births reported by respondents who were not (X2=16.77, d.f. = 1, p